Pillow Breccias Research Articles

Remnants of an Early Archaean (>3.75 Ga) sea-floor, hydrothermal system in the Isua Greenstone Belt

Localised low-strain domains in the ∼3.75 Ga Isua Greenstone Belt, West Greenland, contain a wealth of well-preserved primary structures. These include pillow lavas with dark, fine-grained cooling rims, pillow breccias, debris flows and conglomerates. Here we describe quartz globules from a relatively undeformed pillow breccia, which contains unstrained quartz. Most quartz globules contain only few quartz grains, but some of the larger ones preserve a subidiomorphic rim reminiscent of former geodic lining. The globules are associated with a complex system of quartz veins. Metamorphic conditions recorded in pelites associated with the pillow breccia represent the early stage of a prograde event at 460–480 °C and ∼4 kb. Both globules and quartz veins pre-date the metamorphic fabric, and are interpreted as remnants of a sea-floor type hydrothermal system, operating at the time of lava eruption and pillow formation at ∼3.75 Ga. The globules, which are interpreted as former gas vesicles filled with quartz (and some carbonate), are embedded in an altered basaltic matrix now comprising biotite, muscovite and quartz±tourmaline and carbonate. Silica-filling in the vesicles was contemporaneous with the formation of an intricate hydrothermal vein system. During deformation, the strain was partitioned into the mica-rich rock matrix, when the vesicles behaved as competent objects, eventually cutting or deforming the thinner veins. Amphibolite-facies metamorphic overprint involved a limited flattening of some vesicles and the development of a very faint, sporadic schistosity, preserving most of the pre-metamorphic hydrothermal features. Major metamorphic structures correspond to quartz annealing, yielding curved grain boundaries and triple point junctions. We report rare primary fluid inclusions from quartz in the vesicles. The inclusions contain remnants of two independent fluid/mineral systems comprising pure methane and highly saline aqueous fluids (about 25 wt.% NaCl equivalent), and co-precipitating carbonates (calcite). Especially for the aqueous system, these fluids bear a strong resemblance to present-day sea-floor hydrothermal fluids. They are interpreted as relics of the sea-floor hydrothermal fluid, responsible for the alteration of the pillow fragments and co-precipitation of quartz and carbonate.

Subduction-margin Assemblages in the Proterozoic Pinal and Cochise blocks, southeastern Arizona

Protoliths of Paleoproterozoic rocks broadly referred to as “Pinal Schist” in the Johnny Lyon Hills and the Dragoon and Little Dragoon Mountains of Cochise County, Arizona, include intrusive and subaqueous extrusive mafic rocks, turbiditic clastic rocks, and highly deformed sedimentary rocks visually similar to Phanerozoic mélanges. The combination of these lithologies and their present spatial distribution—clastic rocks to the northwest separated by a region of mélange from mafic igneous rocks to the southeast—is suggestive of an assemblage formed at a subduction-type continental margin. Protoliths of the mafic rock sequence, about 700 m thick, consist upward from its base of composite diabase dikes, pillow lavas, and pillow breccias, and pelitic sedimentary rocks; it may be an ophiolite fragment. The clastic rocks, though complexly deformed, form a largely coherent, mostly northwest-younging sequence. A fault-bounded deformed belt, present between the mafic and the clastic rocks, includes pre-metamorphic mélange and broken formation. The study area falls along or near previously suggested boundaries between lithotectonic assemblages within the Precambrian rocks of southeastern Arizona. Our results provide insight into the location and nature of the boundary and strongly suggest the presence of a suture zone juxtaposing predominantly metavolcanic rocks to the southeast against predominantly metasedimentary rocks to the northwest. Consistent with the terminology used by Eisele and Isachsen (this issue), we adopt the terms Cochise block and Pinal block for the regions to the southeast and northwest of the suture, respectively.

Depositional setting and paleogeographic implications of earth's oldest supracrustal rocks, the >3.7Ga Isua Greenstone belt, West Greenland

Abstract New structural and stratigraphic mapping in the Isua greenstone belt, West Greenland has revealed that the exposed lithologic succession is quite different to that depicted in earlier studies. The key to a better understanding of the stratigraphy has been the recognition of intense strain and metasomatic effects combined with ductile fault structures that segment the belt into a number of tectonic slices. In some of these slices, deformation and metasomatism are somewhat lower than in surrounding slices (though still significant), which permit the recognition of primary depositional features that may be used to compare with lithologies from elsewhere in the belt. The belt is dominated by amphibolite that in a number of places show well-defined pillows. Pillow breccias and basaltic debris flows also occur within this package. Strongly recrystallized ultramafic bodies that occur in the belt are interpreted as intrusions or komatiitic flows. The most common sedimentary rock type is chert/banded iron-formation. These lithologies have been strongly affected by brittle and ductile deformation in combination with coarse recrystallization. Siliciclastic detrital rocks such as conglomerate and sandstone are much less common in the belt, and where present, have intrabasinally derived sources. Highly deformed quartzo-feldspathic schist crops out in a number of places in the belt and most likely has multiple origins. Previously these schists have been considered to be felsic volcaniclastic rocks, though recent mapping, and geochemical, and isotopic studies support the hypothesis that at least some of these schists are highy deformed and carbonated tonalitic gneiss sheets or replaced pillow-lava successions. Carbonate rocks in the belt are now considered to be mostly, or entirely, replacement in origin, where metasomatizing fluids have particularly utilized amphibolite–chert contacts; additionally, some carbonate may represent the products of early sea-floor alteration. We envision a temporarily emergent, below wave base, sea-floor origin for the succession based on the assemblage of lithologies.

Volcano-sedimentary processes operating on a marginal continental arc: the Archean Raquette Lake Formation, Slave Province, Canada

Abstract The 200-m thick, volcano-sedimentary Raquette Lake Formation, located in the south-central Archean Slave Province, represents a remnant arc segment floored by continental crust. The formation overlies the gneissic Sleepy Dragon Complex unconformably, is laterally interstratified with subaqueous mafic basalts of the Cameron River volcanic belt, and is considered the proximal equivalent of the turbidite-dominated Burwash Formation. A continuum of events associated with volcanism and sedimentation, and controlled by extensional tectonics, is advocated. A complex stratigraphy with three volcanic and three sedimentary lithofacies constitute the volcano-sedimentary succession. The volcanic lithofacies include: (1) a mafic volcanic lithofacies composed of subaqueous pillow–pillow breccia, and subaerial massive to blocky flows, (2) a felsic volcanic lithofacies representing felsic flows that were deposited in a subaerial environment, and (3) a felsic volcanic sandstone lithofacies interpreted as shallow-water, wave- and storm-reworked pyroclastic debris derived from explosive eruptions. The sedimentary lithofacies are represented by: (1) a conglomerate–sandstone lithofacies consistent with unconfined debris flow, hyperconcentrated flood flow and talus scree deposits, as well as minor high-energy stream flow conglomerates that formed coalescing, steep-sloped, coarse-clastic fan deltas, (2) a sandstone lithofacies, interpreted as hyperconcentrated flood flow deposits that accumulated at the subaerial–subaqueous interface, and (3) a mudstone lithofacies consistent with suspension sedimentation in a small restricted lagoon-type setting. The Raquette Lake Formation is interpreted as a fringing continental arc that displays both high-energy clastic sedimentation and contemporaneous effusive and explosive mafic and felsic volcanism. Modern analogues that develop along active plate margins in which continental crust plays a significant role include Japan and the Baja California peninsula.

Cyclic volcanic stratigraphy in a late ordovician marginal basin, West Norwegian Caledonides

The well-preserved extrusive sequence of the Solund-Stavfjord Ophiolite Complex (SSOC) in the West Norwegian Caledonides enables reconstruction of the uppermost oceanic crust that developed in a marginal basin. Basaltic sheet flows, pillow lavas and volcanic breccias are the main components of the extrusive sequence and show stratigraphic and structural evidence for a cyclic development. The first stage in a volcanic cycle is characterized by high extrusion rates yielding sheet flows, commonly with the thickest flow units at the base. Sequences of sheet flows can be correlated laterally for at least 6.5 km. Pillow lavas succeed the sheet flows later in a volcanic cycle with progressively smaller pillows forming at decreasing extrusion rates. Volcanic breccias occur towards the end of a volcanic cycle, but may also occur at lower stratigraphie levels. They are made generally of pillow breccias and hyaloclastites. The extrusive sequence of the SSOC oceanic crust was constructed through seven volcanic cycles that resulted in stratigraphic units with thicknesses ranging from 40 to 225 m. This architecture is comparable to sequences in in situ oceanic crust developed along slow- to intermediate-spreading ridges.

The Submarine Volcanic System of the Errigoiti Formation (Albian-Santonian of the Basque-Cantabrian Basin, Northern Spain): Stratigraphic Framework, Facies, and Sequences

ABSTRACT The Errigoiti Formation, located in the central domain of the Basque-Cantabrian basin, is an Albian to Santonian bathyal submarine volcanic system of alkali basaltic character. These volcanics occurred in an extensional geodynamic context generated by the drifting of the Iberian plate with respect to the European plate. Stratigraphic analyses reveal the facies distribution and depositional architecture of a longitudinal cross section of the Errigoiti volcanic system. Four facies groups were distinguished: sheet lavas (SL), derived from low-viscosity lava flows at high magma discharge rate; pillow lavas (PL), derived from intermediate-viscosity lava flows at moderate discharge rate; pillow breccias (PB), derived either from in situ fragmentation of lavas (PB1) or rock-falling from pillow-lava slopes (PB2); and stratified volcaniclastites (VC), derived from volcaniclast resedimentation through gravity flows, either by early remobilization (VC1) or post-volcanic reworking (VC2). The facies groups occur in a predictable repetitive succession, essentially from base to top, an SL-PL-PB-VC stacking pattern. This is interpreted to reflect an upward waning in the magma discharge rate. Five major volcanic sequences make up the Errigoiti Formation. Each comprises packages tens to hundreds of meters thick, deposited during short time spans of volcanic activity separated by long time intervals of volcanic inactivity. Consequently, boundaries between sequences are associated with marked hiatuses, reworked volcaniclasts, and/or turbidite-pelagic sedimentation. Analysis of lateral and vertical facies evolution enabled us to propose a depositional model consisting of three depositional environments. From proximal to distal settings, these are: amalgamated pillow volcanoes, scattered pillow volcanoes, and volcanic fringe, basically responding to a proximal to distal PL/PB, PL/VC, and SL/VC evolution of facies associations. Contemporaneous fossiliferous turbidite-pelagic sequences that surround the volcanic succession allow paleobathymetry to be estimated on the basis of biofacies. Sites with the highest rate of volcanic emission correspond to an average depth of 900 m; sites with thinner volcanic pile accumulated at a minimum depth of 1100 m. The facies sequence established in this work and the processes involved in its formation are comparable with the sequences and processes described in other submarine basic volcanic systems. Consequently, the depositional model for the Errigoiti Formation can perhaps serve as a reference model for this type of volcanic system.

Protoliths of the 3.8–3.7 Ga Isua greenstone belt, West Greenland

The Isua greenstone belt ( Fig. 1) contains the oldest known, relatively well preserved, metavolcanic and metasedimentary rocks on Earth. The rocks are all deformed and many were substantially altered by metasomatism, but both the deformation and metasomatism were heterogeneous. Transitional stages can be seen from relatively well preserved primary volcanic and sedimentary structures to schists in which all primary features have been obliterated. Likewise different kinds, and different episodes, of metasomatic alteration can be seen that produced a diversity of different compositions and metamorphic mineral assemblages from similar protoliths. New geological mapping has traced out gradations between the best preserved protoliths and their diverse deformed and metasomatised equivalents. By this means, the primary nature of the schists that make up most of the Isua greenstone belt was reinterpreted, and a new map that better portrays the primary nature of the rocks has been produced. The previously mapped stratigraphy was found to be of little value in understanding the geology. Stratigraphic units were defined by different and diverse criteria, such as current composition, structure, metamorphic texture, and inferred protoliths. Much of this stratigraphy represents a misinterpretation of the primary nature of the rocks. The new work indicates that most of the Isua greenstone belt consists of fault-bounded rock packages, mainly derived from basaltic and high-Mg basaltic pillow lava and pillow lava breccia, chert–BIF, and a minor component of clastic sedimentary rocks derived from chert and basaltic volcanic rocks. A previously mapped, extensive, unit of felsic volcanic rocks was found to be derived from metasomatised basaltic pillow lava and pillow breccia intruded by numerous sheets of tonalite.

Processes in High-Mg, High-T Magmas: Evidence from Olivine, Chromite and Glass in Palaeogene Picrites from West Greenland

Uncontaminated volcanic rocks from the 60 Ma Vaigat Formation, West Greenland, contain 6·5–30 wt % MgO, averaging 15·5 wt % MgO. Olivine (mg-number 77·4–93·3) forms diverse assemblages of zoned phenocrysts and xenocrysts showing evidence for equilibrium and fractional crystallization, oxidation, partial to complete re-equilibration, as well as magma mixing. The olivine crystals contain glass inclusions and have high contents of Ca and Cr, indicating that all olivines with up to mg-number 93·0 crystallized from melts. Associated chromites (mg-number 45·4–77·2) are essentially unzoned and in equilibrium with the olivines. Matrix glasses from pillow breccias have 6·7–8·8 wt % MgO and quenched close to 1200°C with oxidation states one log-unit above the NNO (nickel–nickel oxide) buffer. Compositional differences between the glasses from different volcanic members are inherited from the primary melts. The magmas erupted as crystal-charged melts, and liquids with more than ∼14 wt % MgO were not erupted. The compositions of the unerupted parental melts were calculated by stepwise addition of equilibrium olivine to the matrix glasses, and these melts had 20–21 wt % MgO and liquidus temperatures of 1515–1560°C. They had lower FeO* than the erupted rocks with 20–21 wt % MgO because the rocks contain more iron-rich olivine than the melts would have had. The accumulated primary melts ascended through a lithospheric lid of ∼100 km thickness, and we envisage that major crystallization took place from ∼45 km depth and to the surface. Magma batches ascended in narrow dyke-like conduits and fractionated high-Mg olivine and chromite at deep levels and less magnesian crystals at shallower levels. At high ascent velocities numerous olivine crystals were carried in suspension to the surface, even some from deep levels, whereas during slower ascent most olivines settled out before eruption. Pulsating ascent rates led to mixing of magma batches in various stages of fractionation. On average, 13 wt % olivine was left within the crust, presumably as olivine-plated conduit walls. The conduit systems are similar to the crystal-rich narrow magma chambers suggested for mid-ocean ridges but are of much greater vertical extent.

Recognizing distinct portions of seamounts using volcanic facies analysis: examples from the Archean Slave Province, NWT, Canada

Archean volcanic rocks in the mafic-dominated, ca. 2.66–2.69 Ga Point Lake and Beaulieu River belts, Slave Province, Northwest Territories, are significant in demonstrating the facies that characterize specific portions of pillow volcanoes or seamounts, irrespective of tectonic setting. Three distinct localities mapped in detail display facies consistent with: (1) proximal, deep-water, (2) medial to distal, deep-water, and (3) medial, shallow-water seamount settings. The proximal facies in the Point Lake belt include a 55-m-thick, non-vesicular pillowed sequence cut by numerous mafic dykes and sills. Dykes contain multiple chilled margins, indicating successive magma pulses which contributed to edifice construction. Abundant feeder conduits, in addition to the absence of fragmental facies and vesicles, are typical of the central, deep water portion of seamounts where growth is initiated. The medial to distal, deep water facies in the Point Lake belt are represented by a 30–80 m-thick assemblage of disorganized pillow breccia, and pillowed and massive flows with 5–27% vesicularity. Massive, non-vesicular hyaloclastite intermingled with sedimentary material (fluidal peperite), in addition to thin shale units interstratified with pillow breccia and hyaloclastite, indicate that sedimentation and volcanism were contemporaneous. An increase in fragmental units and vesicularity relative to the proximal, deep water facies is suggestive of the medial to distal part of a seamount in shallower water. Bedded tuffs, laterally along strike with massive flows, are the results of turbidity current deposition immediately following localized subaqueous eruptions. A medial, shallow water seamount setting is represented in the Beaulieu River belt, by a 5–85 m-thick sequence of vesicular lobate-pillowed and massive flows, stratified pillow breccia and hyaloclastite, and mafic dykes. Vesicularity ranges from 21–49% in pillowed flows, 5–40% in massive flows, and 20–35% in pillow breccia and hyaloclastite. Stratified pillow breccia developed along steep flow fronts in shallow water whereas bedded hyaloclastite formed during reworking and redeposition of autoclastic hyaloclastite on seamount flanks in shallow water. The volcanic facies associations in the study areas are analogous to those of modern seamounts associated with the Mid-Atlantic Ridge and East Pacific Rise, as well as Mesozoic-Cenozoic seamounts in the Canary Islands, Fiji, southwest Japan, the Sea of Japan, and Cyprus. Volcanological studies in the Point Lake and Beaulieu River volcanic belts and subsequent comparisons with Phanerozoic analogues, demonstrate the manner in which distinct portions of ancient seamounts can be recognized in similar Archean terranes.

Evolution of a submerged composite arc volcano: volcanology and geochemistry of the Normétal volcanic complex, Abitibi greenstone belt, Québec, Canada

The 4 km-thick Archean Normétal volcanic complex (NVC), composed of basaltic andesite, dacite, and rhyolite, is represented by five distinct volcanic phases and one sedimentary phase. Initial volcanic construction features effusive mafic volcanism characterized by massive, pillowed and pillow breccia flows and local massive dacite (phases 1 and 2a). Prominent felsic volcanism of phase 2 commences locally with tuffs, lapilli tuffs and lapilli tuff breccias derived either from hydroclastic or autoclastic fragmentation processes (phase 2b). The principal constructive phase of the NVC (phase 2c) is composed of pillowed andesite, massive dacite, and dominant massive, flow banded and lobate rhyolite flows. Autoclastic or hydroclastic brecciation of the former have produced rhyolitic tuff, lapilli tuff and lapilli tuff breccia. Rhyolitic volcanism continued with eruption of lava flows (phase 3) and the intrusion of dykes and felsic endogenous domes (phases 3 and 4). A subsequent 20–70 m-thick sedimentary unit, composed of volcaniclastic turbidites and pelagic background sediments, constitutes a marker horizon indicating volcanic quiescence. Renewed volcanism of phase 5 is characterized by mafic to felsic turbiditic lapilli tuffs and tuffs, and mafic to felsic flows or intrusions. The felsic lapilli tuffs, tuffs and flows host the Normétal VMS deposit. The geometry and volcanic stratigraphy of the NVC suggests emission of viscous, phenocryst-rich felsic flows from three principal centers, including a parasitic western vent, the major central 6 km-wide cauldron structure and an eastern vent. Voluminous viscous felsic lava over a large area supports the inference of numerous vents whereby individual centers coalesced to produce a composite or complex stratovolcano. Proximal to distal facies changes, variable rhyolitic unit and lobe closures argue for multiple conduits. The VMS deposits are located at the western edge of the central cauldron. Geochemical analyses show two complete compositional spectrums (phases 1, 2, 4 and 5) from basaltic andesite to rhyolite. The Zr/Y and LaN/YbN ratios of phases 1, 2, 4 and 5 show a transitional affinity whereas phase 3 is tholeiitic to slightly transitional. Multi-element diagrams suggest that all phases are consistent with subduction-related processes. The mafic-felsic NVC, a composite volcano that formed upon a shield type volcano, displays subaqueous effusive dominant volcanic construction at depth below storm wave base, as indicated by pillowed flows, turbiditic and pelagic sedimentary rocks, and massive sulphide deposits. Geochemistry and physical volcanology of the NVC are consistent with construction of an immature arc volcano. The submerged Izu-Bonin arc volcanoes may be modern analogues.

Submarine geology of the Hilina slump and morpho-structural evolution of Kilauea volcano, Hawaii

Marine geophysical data, including SEA BEAM bathymetry, HAWAII MR1 sidescan, and seismic reflection profiles, along with recent robot submersible observations and samples, were acquired over the offshore continuation of the mobile Kilauea volcano south flank. This slope comprises the three active hot spot volcanoes Mauna Loa, Kilauea, and Loihi seamount and is the locus of the Hawaiian hot spot. The south flank is the site of frequent low-intensity seismicity as well as episodic large-magnitude earthquakes. Its sub-aerial portion creeps seaward at a rate of approximately 10 cm/year. The Hilina slump is the only large submarine landslide in the Hawaiian Archipelago thought to be active, and this study is one of the first to more highly resolve submarine slide features there. The slump is classified into four distinct zones from nearshore to the island’s base. Estimates of size based on these data indicate a slumped area of 2100 km2 and a volume of 10,000–12,000 km3, equivalent to about 10% of the entire island edifice. The overall picture gained from these data sets is one of mass wasting of the neovolcanic terrain as it builds upward and seaward, though reinforcement by young and pre-Hawaii seamounts adjacent to the pedestal is apparent. Extensive lava delta deposits are formed by hyaloclastites and detritus from recent lava flows into the sea. These deposits dominate the upper submarine slope offshore of Kilauea, with pillow breccia revealed at mid-depths. Along the lower flanks, massive outcrops of volcanically derived sedimentary rocks were found underlying Kilauea, thus necessitating a rethinking of previous models of volcanic island development. The morphologic and structural evolutionary model for Kilauea volcano and the Hilina slump proposed here attempts to incorporate this revelation. A hazard assessment for the Hilina slump is presented where it is suggested that displacement of the south flank to date has been restrained by a still developing northeast lateral submarine boundary. When it does fully mature, the south flank may be more subject to land slips triggered by large, long duration earthquakes and thus Kilauea may undergo more frequent episodes of failure with increased displacements.

Strontium and oxygen isotopic constraints on fluid mixing, alteration and mineralization in the TAG hydrothermal deposit

Strontium- and oxygen-isotopic measurements of samples recovered from the Trans-Atlantic Geotraverse (TAG) hydrothermal mound during Leg 158 of the Ocean Drilling Program provide important constraints on the nature of fluid–rock interactions during basalt alteration and mineralization within an active hydrothermal deposit. Fresh Mid-Ocean Ridge Basalt (MORB), with a 87 Sr/ 86 Sr of 0.7026, from the basement beneath the TAG mound was altered at both low and high temperatures by seawater and altered at high temperature by near end-member black smoker fluids. Pillow breccias occurring beneath the margins of the mound are locally recrystallized to chlorite by interaction with large volumes of conductively heated seawater (>200°C). The development of a silicified, sulfide-mineralized stockwork within the basaltic basement follows a simple paragenetic sequence of chloritization followed by mineralization and the development of a quartz+pyrite+paragonite stockwork cut by quartz–pyrite veins. Initial alteration involved the development of chloritic alteration halos around basalt clasts by reaction with a Mg-bearing mixture of upwelling, high-temperature (>300°C), black smoker-type fluid with a minor (<10%) proportion of seawater. Continued high-temperature (>300°C) interaction between the wallrock and these Mg-bearing fluids results in the complete recrystallization of the wallrock to chlorite+quartz+pyrite. The quartz+pyrite+paragonite assemblage replaces the chloritized basalts, and developed by reaction at 250–360°C with end-member hydrothermal fluids having 87 Sr/ 86 Sr ≈0.7038, similar to present-day vent fluids. The uniformity of the 87 Sr/ 86 Sr ratios of hydrothermal assemblages throughout the mound and stockwork requires that the 87 Sr/ 86 Sr ratio of end-member hydrothermal fluids has remained relatively constant for a time period longer than that required to change the interior thermal structure and plumbing network of the mound and underlying stockwork. Precipitation of anhydrite in breccias and as late-stage veins throughout most of the mound and stockwork, down to at least 125 mbsf, records extensive entrainment of seawater into the hydrothermal deposit. 87 Sr/ 86 Sr ratios indicate that most of the anhydrite formed from ≈2:1 mixture of seawater and black smoker fluids (65%±15% seawater). Oxygen-isotopic compositions imply that anhydrite precipitated at temperatures between 147°C and 270°C and require that seawater was conductively heated to between 100°C and 180°C before mixing and precipitation occurred. Anhydrite from the TAG mound has a Sr–Ca partition coefficient K d≈0.60±0.28 (2 σ). This value is in agreement with the range of experimentally determined partition coefficients ( K d≈0.27–0.73) and is similar to those calculated for anhydrite from active black smoker chimneys from 21°N on the East Pacific Rise. The δ 18 O SO 4 of TAG anhydrite brackets the value of seawater sulfate oxygen (≈9.5‰). Dissolution of anhydrite back into the oceans during episodes of hydrothermal quiescence provides a mechanism of buffering seawater sulfate oxygen to an isotopically light composition, in addition to the precipitation and dissolution of anhydrite within the oceanic basement during hydrothermal recharge at the mid-ocean ridges.

Recognizable primary volcanic and sedimentary features in a low‐strain domain of the highly deformed, oldest known (≈ 3.7–3.8 Gyr) Greenstone Belt, Isua, West Greenland

A low‐strain domain has been identified in the metamorphosed, mostly highly deformed volcanic and sedimentary rocks of the early Archaean Isua supracrustal belt. This domain contains well‐preserved volcanic and sedimentary features, including basaltic pillow lavas, pillow breccia, heterogeneous volcanic breccia, amygdules in metabasalt, and polymict conglomerate dominated by recrystallized chert and volcanic clasts. The low‐strain domain is bounded by highly deformed rocks mostly derived from basalt, chert, and banded iron formation. These discoveries demonstrate that some primary features have escaped the pervasive metasomatism dominant in other parts of the belt and, furthermore, strengthen the characterization of the Isua supracrustals as a typical greenstone belt.

The Late Cretaceous Duarte Complex and Siete Cabezas Formation (Hispaniola): Caribbean Oceanic Plateau

The Core of the Central Cordillera in Dominican Republic is composed of oceanic crustal fragments which form three distinct assemblages: the Duarte complex, the Siete Cabezas Formation and the serpentinized ultramafic belt. Three units are recognized in the Duarte complex: (i) basalts interbedded with Late Jurassic ribbon cherts; (ii) picrites and ankaramites, (iii) amphibolites and gneissic amphibolites. The Siete Cabezas Formation is composed of pillow basalts and pillow breccias associated with Upper Cretaceous pelagic sediments. Finally, the serpentinites are intruded by deformed diabase dykes. The Upper Jurassic pillow basalts and diabase dykes have major, trace element and lead isotopic compositions that are consistent with those of NMORB. However, the basalts differ from typical NMORB by lower ~Nd ratios which plot within the range of Ocean Island Basalts (OIB). The basalts and massive diabase of the Rio Verde complex are geochemically similar to the Upper Jurassic basalts and dykes. Thus, these rocks likely represent remnants of the Caribbean Jurassic oceanic crust formed from an oceanic ridge possibly close to a hotspot. The Duarte picrites and olivine-free ankaramites are generally intensely deformed and metamorphosed to greenschist facies. When devoided of any deformation, these rocks show preserved igneous textures and mineralogy with augite-diopside sometimes rimmed by Mg-hastingsite. Ar-Ar ages on separate Mg-hastingsite gave 86 4-1.4 Ma. This age is perfectly consistent with those of the basalts drilled from the Caribbean floor. The Duarte rocks are enriched in Nb-, Ta-, Ti-, and LREE like the Dumisseau tholeiites (Haiti) and the enriched basalts of Gorgona. Their eNd(T 86Ma) ratios range from +5.7 to 8.3 and are similar to those of OIB. Their eSr ratios (-17.7 to -4.6) are less homogeneous due to the hydrothermal alteration. Finally, their Pb isotopic ra t ios [(2~176 = 18.966-19.537] reflect the contribution of an HIMU-like enriched component. These isotopic ratios show that the Duarte picrites and the ankaramites are isotopicallyt similar to mafic lavas from previously described occurences of the Caribbean plateau (Costa Rica, Ha'it, Curagao and Gorgona). The amphibolites and gneissic amphibolites are composed of hornblende + plagioclase + epidote 4quartz. Most of the rocks have flat

Evolution of the northwestern Iblean Mountains, Sicily: uplift, Plicocene/Pleistocene sea-level changes, paleoenvironment, and volcanism

The Late Miocene to Pleistocene evolution of the northwestern Iblean plateau (Sicily) is marked by a complex interplay of subaerial and submarine volcanism, subsidence and uplift, eustatic sea-level changes, and shallow-water carbonate and clay sedimentation. Volcanic activity occurred in distinct phases, differing drastically in volume, chemical composition, eruptive and depositional sites, and eruptive mechanisms. Six of the newly defined formations in the northwestern Iblean plateau are either entirely volcanic or contain significant amounts of volcanics. The eastern part of a shallow marine basin was filled completely by Late Pliocene tholeiitic lava flows (Militello Formation) that had advanced subaerially from the south–southeast. Lava deltas advanced southwestward on top of earlier pillow breccia debris flow deposits intertongued with soft Trubi marls and chalks. True submarine eruptions (Monte Caliella Formation) simultaneously produced densely packed pillow piles up to 250 m thick. Inferred water depths based on volcanologic and paleoecologic criteria of interbedded and overlying calcarenites agree well.

Propagating rift tectonics of a Caledonian marginal basin: Multi-stage seafloor spreading history of the Solund-Stavfjord ophiolite in western Norway

The Late Ordovician Solund-Stavfjord ophiolite in western Norway represents a remnant of the Iapetus oceanic lithosphere that developed in a Caledonian marginal basin. The ophiolite contains three structural domains that display distinctively different crustal architecture that reflects the mode and nature of magmatic and tectonic processes operated during the multi-stage seafloor spreading evolution of this marginal basin. Domain I includes, from top to bottom, an extensive extrusive sequence, a transition zone consisting of dike swarms with screens of pillow breccias, a sheeted dike complex, and plutonic rocks composed mainly of isotropic gabbro and microgabbro. Extrusive rocks include pillow lavas, pillow breccias, and massive sheet flows and are locally sheared and mineralized, containing epidosites, sulfide-sulfate deposits, Fe-oxides, and anhydrite veins, reminiscent of hydrothermal alteration zones on the seafloor along modern mid-ocean ridges. A fossil lava lake in the northern part of the ophiolite consists of a >65-m-thick volcanic sequence composed of a number of separate massive lava units interlayered with pillow lavas and pillow breccia horizons. The NE-trending sheeted dike complex contains multiple intrusions of metabasaltic dikes with one- and two-sided chilled margins and displays a network of both dike-parallel normal and dike-perpendicular oblique-slip faults of oceanic origin. The dike-gabbro boundary is mutually intrusive and represents the root zone of the sheeted dike complex. The internal architecture and rock types of Domain I are analogous to those of intermediate-spreading oceanic crust at modern mid-ocean ridge environments. The ophiolitic units in Domain II include mainly sheeted dikes and plutonic rocks with a general NW structural grain and are commonly faulted against each other, although primary intrusive relations between the sheeted dikes and the gabbros are locally well preserved. The exposures of this domain occur only in the northern and southern parts of the ophiolite complex and are separated by the ENE-trending Domain III, in which isotropic to pegmatitic gabbros and dike swarms are plastically deformed along ENE-striking sinistral shear zones. These shear zones, which locally include fault slivers of serpentinite intrusions, are crosscut by N20°E-striking undeformed basaltic dike swarms that contain xenoliths of gabbroic material. The NW-trending sheeted dike complex in the northern part of Domain II curves into an ENE orientation approaching Domain III in the south. The anomalous nature of deformed crust in Domain III is interpreted to have developed within an oceanic fracture zone or transform fault boundary. REE chemistry of representative extrusive and dike rocks from all three domains indicates N- to E-MORB affinities of their magmas with high Th/Ta ratios that are characteristic of subduction zone environments. The magmatic evolution of Domain I encompasses closed-system fractional crystallization of high-Mg basaltic magmas in small ephemeral chambers, which gradually interconnected to form large chambers in which mixing of primary magmas with more evolved and fractionated magma caused resetting of magma compositions through time. The compositional range from high-Mg basalts to ferrobasalts within Domain I is reminiscent of modern propagating rift basalts. We interpret the NE-trending Domain I as a remnant of an intermediate-spread rift system that propagated northeastwards (in present coordinate system) into a pre-existing oceanic crust, which was developed along the NW-trending doomed rift (Domain II) in the marginal basin. The N20°E dikes laterally intruding into the anomalous oceanic crust in Domain III represent the tip of the rift propagator. The inferred propagating rift tectonics of the Solund-Stavfjord ophiolite is similar to the evolutionary history of the modern Lau back-arc basin in the SW Pacific and suggests a complex magmatic evolution of the Caledonian marginal basin via multi-stage seafloor spreading tectonics.

Volcaniclastic processes on modern submarine arc stratovolcanoes: sidescan and photographic evidence from the Rumble IV and V volcanoes, southern Kermadec Arc (SW Pacific)

Studies of volcaniclastic processes on modern, submarine, arc stratovolcanoes are rare. Facies models of submarine volcanism and associated volcaniclastic deposits have to date emphasized general and/or regional processes. Two andesite-alkali basaltic volcanoes (Rumble IV and V volcanoes) from the southern Kermadec arc are “mapped” with swath bathymetry and sidescan imagery, and photographic data, to characterise volcaniclastic processes on these modern, submarine, arc stratovolcanoes, and their interaction with ambient marine processes. The principal observation from the Rumble IV and V data is the fine-scale spatial variability in the distribution of effusive lavas, various volcaniclastic deposits, and hemipelagic-pelagic sediments. Mass-movement of volcaniclastic deposits is common, and includes (1) epiclastic debris flows, (2) pillow breccia talus deposition penecontemporaneous with lava eruption, (3) traction particulate gravity flows, and possibly (4) sector collapse. Epiclastic redeposition is inferred to be mobilised by episodic, but frequent, localised, small volume flows, which largely redeposit debris on the edifice. Winnowed and rippled volcaniclastic sediment indicates bottom-current activity is common, but localised, over the entire height of both volcanoes, down to a water depth of ~ 2200 m. These currents also possibly entrain detritus for subsequent mobilisation by particulate gravity flows. Irregular and ragged spatter agglutinates on Rumble IV, interpreted as evidence of lava fountaining, and wellsorted, fines poor, scoriaceous material on Rumble V, interpreted as evidence of explosive eruption, indicate the transition between effusive and explosive volcanism is between 650 and 700 m water depth. Other southern Kermadec volcanoes (e.g. Tangaroa) have scoriaceous detritus at 960–970 m water depth. Positions of these effusive-explosive transition zones are likely to oscillate up and down the volcano during glacioeustatic fluctuations.

Off-ridge alkaline magmatism and seamount volcanoes in the Masirah island ophiolite, Oman

The Masirah ophiolite offers an unique opportunity to study well preserved small seamount structures. Obducted seamounts have not been described up to now, and from the present-day ocean floor they are almost exclusively known from bathymetric studies. The thin oceanic crust of the Masirah ophiolite was formed at a ridge-transform intersect in Upper Jurassic time. It was overprinted and reworked by a major intra-oceanic tectono-magmatic event at mid-Cretaceous time, that has been well dated owing to the presence of interstratified sedimentary rocks (late Hauterivian to early Barremian, c. 130-125 Ma). This mid-Cretaceous magmatism produced alkaline volcanic rocks ranging in chemistry from alkalibasalts to rhyolites. Volcanism occurred in a NW-SE extensional regime. Small, elongate submarine volcano structures (seamounts) developed within widespread alkalibasaltic pillow lava and pillow breccia deposits, which are interfingered with deep-marine pelagic sediments. The volcanoes reached a maximum of a few kilometres in diameter and a few hundred metres in height. The seamounts are built up of basic to acid subvolcanic stock- or sheet-like intrusions, several generations of dikes, vent agglomerates and pyro- to epiclastic deposits. The latter range from coarse breccias to finely stratified lapilli and record explosive volcanism in a deep marine environment. In the magma chambers under the volcanoes local differentiations to trachytic and rhyolitic members took place. The alkaline rocks show a pronounced ocean island basalt (OIB) character indicating the considerable contribution of a mantle plume source (hotspot). As cause of the volcanism we propose a combination of original transform setting followed by drift past the Marion hotspot during the major plate tectonic reorganization between Greater India, Madagascar and Africa starting in mid-Cretaceous time.

Tholeiitic‐alkalic transition at subglacial volcanoes, Tuya region, British Columbia, Canada

Ash Mountain, South Tuya, and Tuya Butte are three small basaltic volcanoes in the Stikine volcanic belt of northern British Columbia. The volcanoes rise 700, 500, and 400 m above their bases and are about 3.2, 1.6, and 2.6 km3 in volume, respectively. They began eruptive activity under several hundred meters of overlying glacial ice, or water in an ice‐impounded lake, and undegassed pillow lava was erupted and forms the bases of all three. Later, as the vents grew into shallow water, explosive phreatomagmatic activity erupted partly degassed glassy tuffs. Finally, when the volcano emerged through the surface of the ice or water (or the water was drained), degassed subaerial lava flows were erupted and were converted to assemblages of foreset‐bedded pillow breccia and pillow lava when subaerial flows crossed a shoreline and flowed into meltwater lakes. The undegassed subglacial pillow base of Ash Mountain is overlain by partly degassed pillows and hyaloclastite tuff cut by dikes; at South Tuya the pillow base is overlain by hyaloclastite tuffs and lenses of pillow lava; at Tuya Butte the pillow base is overlain by foreset‐bedded pillow lava, pillow breccias, and hyaloclastite tuffs, which in turn are overlain by subaerial lava flows composing a small shield volcano. The undegassed basal subglacial pillow lava of the three volcanoes contain 0.10 ± 0.01 wt % sulfur and ∼0.5 wt % H2O. The overlying partly degassed assemblages contain 0.06 ± 0.02% sulfur and ∼0.2% H2O at Ash Mountain, 0.07±0.01% sulfur at South Tuya, and 0.03±0.01% sulfur at Tuya Butte. The differences in the degree of degassing can be related to the nature of eruption and quenching and the distance of flow of the subaerial lava. When the volcanoes switched from subglacial to shallow water or subaerial eruptions, as shown by change to more explosive activity and then to subaerial lava flows (and by a marked reduction of sulfur in volcanic glass), the magma shifted from tholeiitic to alkalic composition. This transition occurs at each of the three volcanoes. The tholeiitic and alkalic magmas cannot be related by shallow crystal fractionation and apparently originated by differing degrees of deep melting at a mantle source. Prior to eruption the tholeiitic melts overlay alkalic melts in shallow chambers underlying each of the volcanoes because of their lower density and were, therefore, the first to erupt under subglacial conditions. As the volcano grew through the ice (or ice‐impounded water), the volcanic conduit vented to the atmosphere, producing a partial depressurization of the conduit and the subsurface chamber. This sudden reduction in confining pressure caused enhanced vesiculation of volatile saturated melts, particularly of the more volatile‐rich alkalic melts, causing them to rise to the top of the chamber and erupt.

Intraglacial volcanism in the Wells Gray–Clearwater volcanic field, east-central British Columbia, Canada

Small-volume, subaerial, subaqueous and subglacial basaltic eruptions occurred in the Wells Gray–Clearwater area during Quaternary time. Part of this time, significant thicknesses of glacial ice were present. Dating of intraglacial volcanic features corroborates other evidence of an Early Pleistocene, Cordilleran-wide ice sheet. Of the intraglacial volcanoes investigated, three were studied in detail and of these, two probably erupted during the Fraser glaciation (11–20 ka), when maximum ice level exceeded 2100 m elevation. Major-element and sulphur concentrations were measured in glass from the volcanoes to provide insight into vent conditions at the time of eruption. Hyalo Ridge (2102 m elevation, whole-rock K–Ar age of 0.02 ± 0.01 Ma) is a small volcanic edifice capped by lava flows with coherent pillowed lavas and interbedded hyaloclastite exposed over nearly 400 m altitude on its east flank. Low sulphur content (&lt;0.03 wt.%) in pillow rim glasses indicates that the lavas are degassed. It is interpreted that the vent built above the water (or ice) surface then fed lava flows that crossed a shoreline and produced pillowed flows. Pyramid Mountain is a volcanic cone 240 m high, comprised of glassy, vesicular, lapilli-tuff breccia. The highly alkalic glass contains 0.1 wt.% S (considered high), and indicates a high original volatile content and drastic quenching, probably during phreatomagmatic eruption from a meltwater-flooded vent. East of the Clearwater River a sequence of massive pillowed flows and pillow joint-block breccias is exposed from 880 to 1320 m elevation (0.27 ± 0.05 Ma). The vent location is unknown. Moderate S content (0.040–0.055 wt.%) indicates that the lavas were erupted in shallow water and are largely degassed. The S content of glass in dykes cutting the pillow breccias is low. The dykes are interpreted as lava that has flowed laterally or down into cracks.